The Retry-After header indicates how long the user agent should wait before making a follow-up request. It's commonly used with 503 (Service Unavailable) responses or for rate limiting. The value can be either a number of seconds to wait or an HTTP-date after which to retry. This header helps in managing traffic and server load by instructing clients when to retry their requests.

The type of the message body, specified as a MIME type.

private means the response can only be stored by the browser's cache, but not by CDNs, proxies, or any other shared caches.
Warning no-store means the response may not be stored in any cache, including the browser's cache. Every request will hit the origin server. If this page can be cached, consider using public with an appropriate max-age.
Notice private has no effect when no-store is set since nothing will be cached. It can be safely removed.

Used by the server to advertise its support of partial HTTP requests. The browser can ask for a range or subset of the response body. It's a feature that allows a browser to resume an interrupted download. bytes is the only range unit currently supported. none means that the server does not support range requests.

The date and time at which the request was made. A browser uses it for age calculations rather than using its own internal date and time; e.g. when comparing against Max-Age or Expires.

The Via header tracks how a page is forwarded from proxy to proxy. Beware, not all proxies append themselves to the Via header.

Exposes server-side performance metrics (e.g. database query time, cache lookups, total processing time) to the browser. These metrics appear in the browser's DevTools Network tab, making it easier to diagnose slow responses without needing server-side log access.

The Strict-Transport-Security header (HSTS) instructs browsers to only use HTTPS for future connections to this domain, enhancing security by preventing downgrade attacks and cookie hijacking.
max-age specifies the time, in seconds, that the browser should remember to use HTTPS only for this domain.
includesubdomains instructs the browser that all subdomains are HTTPS-only as well.

The X-Content-Type-Options header prevents browsers from guessing a response's content type. Without it, browsers may interpret files differently than intended, which can lead to security vulnerabilities.
The value nosniff is correctly set.

X-Frame-Options prevents this URL from being embedded in an iframe. This protects against clickjacking attacks.
sameorigin allows this page to be displayed in an iframe on the same origin. This is the recommended setting.

A legacy header that enables the browser's built-in cross-site scripting (XSS) filter. Modern browsers ignore it in favor of Content-Security-Policy.
1 enables XSS filtering.
mode=block blocks the response entirely if an attack is detected, instead of sanitizing the page.
Notice Add a Content-Security-Policy header for more comprehensive protection.

Identifies the software used by the origin server to handle the request (e.g. Apache, Nginx, Cloudflare).
Notice Consider removing or minimizing the Server header. Even without a version number, it reveals the server software, which aids reconnaissance.

The Report-To header defines where browsers should send error reports. It sets up named reporting endpoints that other headers (like NEL and Content-Security-Policy) reference when they need to send violation or failure data.
group defines the reporting group. Groups allow reports to be grouped logically; e.g. there could be a group for network errors and a second group for browser feature deprecation reports.
max_age defines the number of seconds the browser should remember these settings. Prevents the browser from having to parse the JSON on each request.
endpoints defines one or more URLs where the reports need to be sent to. Multiple URLs can be specified for failover and load-balancing. Endpoints can be assigned a weight to distribute load, with each endpoint receiving a specified fraction of the reporting traffic. Endpoints can also be assigned a priority to set up fallback collectors.
include_subdomains is a boolean that enables or disables NEL reporting for subdomains of the origin.

The NEL (Network Error Logging) header tells browsers to report network-level failures (DNS errors, TCP timeouts, TLS problems) back to the server. This means you can detect when real users fail to reach your site, even though the failure happens before your server sees a request.
report_to defines the reporting group that reports for this NEL policy will be sent to. The reporting group details are specified in the Report-To header.
max_age defines the number of seconds the browser should remember these settings. Prevents the browser from having to parse the JSON on each request.
include_subdomains is a boolean that enables or disables NEL reporting for subdomains of the origin.
success_fraction defines the sampling rate that should be applied to reports about successful network requests. Its value must be a number between 0.0 and 1.0: 0 means no successful requests should be reported, and 1 means that every successful request should be reported.
failure_fraction defines the sampling rate that should be applied to reports about failed network requests. Its value must be a number between 0.0 and 1.0: 0 means no failed requests should be reported, and 1 means that every failed request should be reported.

The size of the message body, in bytes.

missing Add a Content-Security-Policy header. The Content-Security-Policy header helps browsers prevent cross site scripting (XSS) and data injection attacks.

missing Add a Referrer-Policy header. When a visitor navigates from one page to another, browsers often pass along referrer information. The Referrer-Policy header controls how much referrer information a browser can share. This is important to configure when private information is embedded in the path or query string and passed onto an external destination.

missing Add a Permissions-Policy header. Restrict access to device features like the camera, microphone, location, accelerometer and much more.

missing Add a Cross-Origin-Embedder-Policy header. It requires cross-origin resources to explicitly consent before this page can load them, protecting those resources from being exposed to Spectre-style timing attacks. Together with Cross-Origin-Opener-Policy, it enables cross-origin isolation and access to SharedArrayBuffer.

missing Add a Cross-Origin-Opener-Policy header. It prevents other sites from retaining a window reference to this page when opened via window.open() or navigation, blocking script-based attacks through shared browsing contexts.

missing Add a Cross-Origin-Resource-Policy header. It controls which origins can embed or load this page's resources (images, scripts, etc.), preventing hotlinking and cross-origin data leaks.

missing Add a X-Permitted-Cross-Domain-Policies header to prevent Flash, Adobe Reader and other clients from sharing data across domains.

missing No Content-Encoding header found. Enable compression such as gzip or br (Brotli) to reduce transfer sizes. Brotli typically achieves 15–20% better compression than gzip and is supported by all modern browsers.